As the storage capacity of magnetic data storage devices increases, the magnetic bit size in the storage media becomes smaller. A smaller bit size requires a reduction in physical dimensions of the recording head.
Magnetic recording heads typically include a reader portion having a sensor for retrieving magnetically encoded information stored on a magnetic disc. Magnetic flux from the surface of the disc causes rotation of the magnetization vector of a sensing layer or layers of the sensor, which in turn causes a change in the electrical properties of the sensor. The sensing layers are often called free layers, since the magnetization vectors of the sensing layers are free to rotate in response to external magnetic flux. The change in the electrical properties of the sensor may be detected by passing a current through the sensor and measuring a voltage across the sensor. Depending on the geometry of the device, the sense current may be passed in the plane (CIP) of the layers of the device or perpendicular to the plane (CPP) of the layers of the device. External circuitry then converts the voltage information into an appropriate format and manipulates that information as necessary to recover information encoded on the disc.
The magnetoresistive (MR) sensor (reader) may be located between high permeability shields to screen the sensor from external magnetic fields, such as stray fields from media. With increasing recording density, the shield-to-shield spacing of the reader becomes narrower to meet higher linear density requirements and to screen external flux.
A narrow shield-to-shield spacing of the reader requires that layers in the reader stack become thinner, which makes the sensor more sensitive to materials around it. A thinner reader stack cap could make the free layer more susceptible to magnetic domain changes in the shields and introduce unwanted instability. Furthermore, the shield topography may also introduce additional domain movement in the shield, which could influence the free layer response and add noise to the read signal.
In order to ensure the stability of the reader when it is subjected to external fields and to minimize noise from any magnetic domain movement inside the shields, it would be desirable to provide improved control of the magnetic properties of the shields.